The present invention pertains to the field of providing fiber Bragg gratings in optical waveguides such as optical fibers. More particularly, the present invention is directed to altering the index of refraction of at least some grating elements or of altering the index of refraction of an optical waveguide so as to provide various kinds of gratings besides conventional Bragg gratings, such as phase-shifted gratings, sampled gratings, long-period gratings and other gratings that provide higher levels of optical functionality.
Fiber Bragg gratings and other grating elements (such as long period gratings used to couple core modes of propagation to cladding modes of propagation) are in widespread use in both telecommunications and sensing applications. In many applications, the grating spectral profile (i.e. the set of wavelengths reflected or transmitted by the grating) is vitally important to the performance of the device. To achieve a specific grating profile, the grating strength (the ultraviolet induced index change for each grating element) can be xe2x80x9capodized,xe2x80x9d i.e. the grating strength can be caused to vary spatially along the grating. Without apodization, a grating has a uniform strength, i.e. the strength is the same for all of its elements. Light reflected from such a grating includes not only wavelengths associated with the inter-element grating distance, but also sidelobes associated with the sharp edges of the grating region (sinc-squared sidelobes for standard unsaturated fiber Bragg gratings). To eliminate the sidelobes, apodization is used to reduce the grating strength smoothly from full strength at the center of the grating to zero strength at the edges of the grating.
Such sidelobes cause problems in many grating applications. In wavelength division multiplexing (WDM) applications, the sidelobes create out-of-band reflectivity, and in sensing applications, the sidelobes exacerbate the task of determining the grating central wavelength.
The prior art teaches using various ultraviolet beam profile shaping or beam modulation techniques for apodization. But such techniques must be performed as part of the original grating fabrication process. In addition, such techniques require sophisticated control mechanisms to achieve the required ultraviolet beam profile shaping or beam modulation.
What is needed is a simple method by which a grating can be thermally treated, post-fabrication, so as to produce an apodized grating with improved spectral characteristics, such as a spectral profile substantially free of sidelobes associated with the grating edges, or even spectral characteristics fundamentally different than those provided as a result of the fabrication process, such as would be achieved by selectively erasing the grating elements in the mid-section of a grating, leaving only the edge elements at full or near full strength, and so creating a cavity for a Fabry-Perot grating filter. Such a technique would clearly be useful in creating various kinds of gratings besides conventional Bragg gratings, such as phase-shifted gratings, long-period gratings, or higher functionality gratings.
Accordingly, the present invention provides a method for erasing some or all of at least some of the grating elements of a grating inscribed in an optical waveguide, such as in the core of an optical fiber, the method comprising the steps of:
providing a source of laser light having a beam intensity, the laser light having a wavelength that is at least partially absorbed by the optical waveguide material; determining a temperature profile suitable for achieving a predetermined desired apodization; determining a target site on the optical waveguide suitable for directing the laser light; determining how long to apply the laser light to the target site, based on the beam intensity, so as to create at least a portion of the temperature profile; and directing the laser light to the target site for the length of time determined to be necessary to create at least a portion of the temperature profile.
In a further aspect of the invention, the laser is a CO2 laser.
In another, further aspect of the invention, a plurality of target sites are determined within the span of the grating and the laser beam is directed to each of the target sites so as to at least partially erase the grating at each target site and so as to provide a smooth transition between each target site and an adjacent target site from a reduced strength at the target site to approximately full strength midway between the target site and the adjacent target site and again to a reduced strength at the adjacent target site, thereby providing a higher-functionality grating.
In yet another, further aspect of the invention, the method further comprises the steps of determining a sweep rate as a function of position relative to the target site for sweeping the laser beam from the target site across a portion of the grating, and sweeping the beam across the portion of the grating at the sweep rate so determined. An equivalent procedure modulates the power of the laser beam while sweeping the laser beam across the portion of the grating at a constant (position-independent) sweep rate.
In still yet another, further aspect of the invention, instead of apodizing a grating in an optical waveguide, the present invention provides a method for creating a large-scale grating, such as a long-period grating or a large-scale aperiodic grating, in an optical waveguide, such as an optical fiber, beginning with a grating length of the optical waveguide over which the index of refraction is made to be substantially constant (i.e. without any appreciable index modulation over the grating length). The method then includes the steps of:
providing a source of laser light having a beam intensity, the laser light having a wavelength that is at least partially absorbed by the optical waveguide material; determining a temperature profile suitable for achieving a predetermined desired apodization suitable for creating a large-scale grating, the temperature profile including a plurality of points along the grating length at which the temperature of the optical waveguide is to reach a maximum value; determining how long to apply the laser light to each of the points along the grating length at which the temperature of the optical waveguide is to reach a maximum value, based on the beam intensity, so as to create the temperature profile within the optical waveguide; and directing the laser light to each of the points along the grating length at which the temperature of the optical waveguide is to reach a maximum value for the length of time determined to be necessary to create the temperature profile within the optical waveguide.